microstructural analysis of the rhode island formation ... · pally the data to provide a visual...
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Undergraduate Review
Volume 2 Article 29
2006
Microstructural Analysis of the Rhode IslandFormation, Narragansett Basin, MAElizabeth Connell
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This item is available as part of Virtual Commons, the open-access institutional repository of Bridgewater State University, Bridgewater, Massachusetts.Copyright © 2006 Elizabeth Connell
Recommended CitationConnell, Elizabeth (2006). Microstructural Analysis of the Rhode Island Formation, Narragansett Basin, MA. Undergraduate Review,2, 224-330.Available at: http://vc.bridgew.edu/undergrad_rev/vol2/iss1/29
22.
Elizabeth Connell conducted her research
with funding from an AlP Summer Grant
under the mentorship of Dr. Michael KroL
TilE lJNDEII:GIlADUATE REVIEW
Microstructural Analysis of theRhode Island Formation,Narragansett Basin, MA
BY ELIZABETH CONNELL
Abstract
he Narragansett Basin is a Pennsylvanian-age transtensional ba
sin that formed within the Avalon Terrane approximately 320
Ma. The Narragansett Basin consists of different stratigraphic
units, with the Rhode Island Formation being the most extensive
(Towe, 1959). Following deposition and lithification, the Rhode Island Forma
tion experienced several phases of intense deformation and metamorphism as
a result of Africa colliding with the eastern margin of North America. The Al
leghanian Orogeny created the supercontinent Pangea, culminating around 280
Ma (Winstch et aI., 1992).
This project investigated the lithologic and microstructural characteristics
of the lower portion of a 1500 foot deep drill core obtained along the eastern
margin of the Narragansett Basin near Somerset, MA. The core consists of a
sequence of alternating layers of sandstone and siltstone, with minor amounts
of coa!.
Microstructural analysis reveals evidence of both low temperature and high
er temperature deformation episodes that affected the Rhode Island Formation.
The low temperature episode is defined by a dominant pressure solution cleav
age and locally developed pressure fibers. The presumably older, high tempera
ture episode is dominated by crystal-plastic deformation that is preserved as
statically recrystallized quartz. Mineral assemblages near the bottom of the core
contain abundant biotite and biotite/chlorite pseudomorphs after garnet. This
suggests metamorphic temperatures in excess of -450-500'C for the lower por~
tion of the core in agreement with the quartz microstructures.
Introduction
This paper examined the lower portion of a drill core
from Somerset, MA (from a depth of 750 feet to a depth
of 1500 feet) obtained along the eastern margin of the Nar
ragansett Basin (Fig. I). We document the microstructures
preserved in rocks of the Rhode Island Formation and iden
tify the types ofdeformation mechanisms that operated dur
ing an intense phase of the Alleghanian Orogeny. Rocks and
minerals respond to deformation in various ways depend
ing on the dominant deformation mechanism. By studying
the microstructures we were able to decipher the type of
mechanism(s) that operated during deformation and meta
morphism. On the basis of the type of microstructures and
mineral assemblages present, we can infer the temperature
pressure conditions that existed during this episode of de
formation.
The predominant deformation mechanism was pres
sure solution. Pressure solution is a selective process that
involves the dissolution, transport, and re-precipitation of
material through an intergranular fluid, in response to high
compressive stresses (Davis and Reynolds, 19%).
Surficial studies have documented the style of deforma
tion that affected rock of the Narragansett Basin mainly in
Rhode Island (e.g. Mosher et al., 1987). Mosher et al. (1987)
showed that rocks in the southern portion of the Narragan
sett Basin in Rhode Island experienced intense deformation
and high grade metamorphism.
Little attention has been paid to the rocks of the Nar
ragansett Basin in eastern Massachusetts and in particular
within the subsurface. Our results have helped to provide
new information concerning the mechanisms and condi
tions of deformation and metamorphism that occurred dur
ing this intense period of tectonic activity. Ultimately our re
sults will help refine current tectonic models for the geologic
evolution of eastern Massachusetts.
125
Geologic setting
The Narragansett Basin is a Pennsylvanian-age basin that
formed within a transtensional tectonic environment approx
imately 320 Ma (Fig. 1). The Narragansett Basin consists ofa
variety of lithostratigraphic units, of which the Rhode Island
Formation is the most extensive in terms of thickness (Towe,
1959). The basin was a locus of sedimentation as material
was transported from the northeast via rivers and streams.
Deposition resulted in an accumulation of >15,000 feet of
clastic sediment. Following deposition and lithification, the
basin experienced several phases of intense deformation and
metamorphism during the Alleghanian Orogeny. This event
was a result of the collision between Africa and the eastern
margin of North America, creating the supercontinent Pan
gea, about 280 Ma (Winstch et aI., 1992).
Methodology
The drill core studied was extracted in Somerset, MA
and is approximately 1500 feet in length. This research proj
ect concentrated on the lower 750 feet ofdrill core, while the
upper 750 feet ofdrill core was examined by a fellow Bridge
water State College student, Ashlee Kirkwood. Lithologies
were logged and a stratigraphic column was constructed
that displays rock type and thickness versus depth (Fig. 2).
During mesoscopic analysis, samples were selected for pe
trographic study based on their appearance and textures.
Samples were cut using a diamond-tipped rock saw and
trimmed into small rectangular chips. After being polished,
these chips were sent to a commercial laboratory where they
were mounted to glass slides and ground to a thickness of
30 microns.
Results
Mesoscopic Lithologic Analysis
The lower portion of the drill core contained a sequence
of alternating layers of sandstone and siltstone, with minor
amounts ofcoal (Fig. 2). Sedimentarybedding is inclined about
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227
Rgure 2 - Generalized stratigraphic column ofSomerset drill COfe.
IlRIOGEW,4,TEJl ST"1'E CO~LEGE
228
Rgure 4 - XPL (4X)
Sample SOM~86~1:
This sample is a very fine-grained siltstone obtained from
a depth of 1317' that contains quartz, biotite, and opaque
grains. Figure 5 shows pressure fibers surrounding an opaque
grain which indicates low temperature metamorphism and
deformation. Biotite is present in figure 6 which is generally
indicative of higher temperature metamorphic conditions.
Therefore, there may have been two distinct temperature
changes or events that occurred to have both characteristics
present in the same sample. One can infer that the rock was
brought up to a very high temperature (-350'C - 500"C) at
which the biotite began to grow and as the rocks were cooled
the pressure fibers developed during compression.
15_20· from the horizontal. Sandstone that is present is typi
caJly fine~grained with siltstone being extremely fine-grained.
Thicknesses ofindividual units ranged from only inches to 10'5
or lOO's of feet. Several calcite veins occur throughout the core
and crosscut sedimentary layering at relatively high angles.
Petrographic Analysis
The following data reflects the mineralogy and micro
structures obtained through petrographic analysis of samples
that provide information pertaining to the geologic evolu
lion of the Narragansett Basin; photomicrographs accom
pallY the data to provide a visual example. XPL = crossed
polarized light; PPL = plane polarized light.
Figure 3 - XPL (4X)
Sample SOM-56-!:
This sample is a fine-grained, well sorted sandstone ob·
tained from a depth of 872' that contains quartz and an abun
dance of opaque grains. Figure 3 shows an example of slaty
cleavage and pressure fibers developed on an opaque grain.
Sample SOM~83·1:
This sample is a fine-grained, well sorted sandstone ob
tained from a depth of 1269' that contains quartz, muscovite,
and biotite. Figure 4 shows an example of well-developed
crenulation cleavage within the mica-rich layers, and consid-
erably less within quartz-rich layers. Figure 5 ~ XPL (lOX)
THE UNDERGRADUATE REVIEW
Figure 6 - XPL (4X)
Sample SOM~96~2:
This sample is a fine-grained, well sorted sandstone ob·
tained from a depth of 1424' that contains quartz, biotite,
and chlorite. Figure 7 shows chlorite pseudomorphs after
garnet that also display an asymmetric shear fabric. In this
sample the low temperature pressure solution overprints the
high temperature fabric. Figure 8 shows chlorite and bio
tite after garnet. The presence of the pseudomorphs in both
Figures 7 and 8 is indicative of retrogression following high
temperature metamorphism.
Figure 7 - PPL (4X)
229
Figure 8 - PPL (lOX)
Sample SOM-96~3
This sample is a fine-grained, well sorted sandstone ob
tained from a depth of 1422' that contains quartz, biotite,
clay, and opaque grains. Figure 9 shows a biotite skeleton that
is breaking down, as well as parasitic folds. The folds bend
around the biotite, indicating that the biotite was there first.
One could infer that the biotite was originally a garnet and
was later converted; after the rocks were heated up to a tem
perature where garnet could grow, they began to cool and the
folds formed on the retrograde side of the cooling path.
Figure 9 - XPL (4X)
BRIDGEWATER STATE COLLEGE
130
Conclusion
The lower portion of the Somerset, MA core preserved
evidence for two distinct thermal events. A high tempera
ture metamorphic event that locally achieved garnet-grade
conditions. and a low temperature deformation episode that
is defined by locally developed pressure fibers and a domi
nant pressure solution cleavage.
After conducting a mesoscopic analysis of the drill core
lithology and a detailed petrographic analysis of the mineral
ogy and microstructures we conclude that the lower portion
of the core (from a depth of -750 feet to a depth of 1500 feet)
experienced both low temperature and high temperature de
formation and metamorphism.
Low temperature deformation is identified by locaUy de
veloped pressure fibers and the presence of a wel.l-developed
crenulation cleavage. The high temperature metamorphism is
recognized by the abundance of biotite as well as biotite/chlorite
pseudomorphs after garnet. In addition. the presence of stati
cally recrystallized quartz also suggests elevated temperatures
persisted after the high temperature metamorphism. The pres
ence of garnet, a metamorphic mineral. suggests that tempera
tures in these rocks exceeded 45O"C - SOO"c.
References Cited:
The high temperature metamorphism is presumably old
er than the low temperature metamorphism. The relative
ages of these events can be established on the basis of the
preserved mineral assemblages and microstructures. The
low temperature event records evidence of pressure solution
and development of a crenulation cleavage. These features
would most likely have been obliterated when subjected to
higher temperatures. However. the biotite/chlorite pseu
domorphs suggest temperatures were high enough to grow
garnet and upon cooling retrograded to biotite/chlorite. It
is unclear whether these events represent separate episodes
of deformation and metamorphism or if they refle<:t a tran
sition from high temperature metamorphism to lower tem
peratures during cooling. but continued deformation.
Acknowledgments
I would like to thank the Adrian Tinsley Program for Un
dergraduate Research for funding this research project and
providing me with this wonderful opportunity. I also thank
my mentor, Dr. Michael A. Krol for all of his time and guid
ance throughout the research process. Finally, I would also
like to thank my family for all of their patience and support.
Davis. G. H.. and S. J. Reynolds. Structural Geology ojRocks and Regions. Brisbane:. John Wiley & Sons. Inc.. 1996.
Mosher. S.• Burks. R.I .• and Reck, B.H. "Alleghanian deformation in the southern Narragansett Basin. Rhode lsland~ Geological Society ojAmerica Centen
nial Field Guide - Northeastern Section 1987.
Towe. K. M. "Petrology and Source of Sediments in the Narragansett Basinof Rhode Island and Massachusetts~ Journal aJSedimentary Petrology 29.4(1959), 503-512.
Winstch, R.P.. Sutter, J.F., Kunk, M.J .• Aleinkoff. J.N.• and Dorias, M.J. ~Con
trasting P-T-t paths: thermochronologic evidence for a late Paleozoic final assembly of the Avalonian composite terrane in the New England Appalachains.~
Tectonics volume 11 (1992): 672-689.
nIH Ufl/l)lIIlGIlAl)UATE REVIEW